Pulp screen



May l0, 1955 w. F. MATHEwsoNl PULP SCREEN 9 Sheets-Sheet l Filed Aprill, 1953 -l w., \\\\\\\I|.PI.H Il .c a o 0 W h mb bb o 4 w CH Q E 1/ n IM Q Wv\ 5 l O F. f llll Q f, iP T 1.! L 0 m 0 O\ ,I n u I'. w 0 Q vm n HA uw n Mm I y Ww S n 0 0 O .l W #l :1LT LJ-, .0.0. x N. O m* Q o U O Gf.. O Q 0 Q on.. NNW Q Q Q Q s o May 10, 1955 w. F. MATHEwsoN 2,707,905

PULP SCREEN Filed April l, 1955 9 Sheets-Sheet 2 May 10, 1955 W. F.MATHEWSON PULP SCREEN 9 Sheets-Sheet 3 Filed April l, 1953 E'Maelumu Mayl0, 1955 w. FMATHEWSON PULP SCREEN Filed April l, 1953 9 Sheets-Sheet 412129223024: Wlfefed I'llizewcons 9 Sheets-Sheet 5 Iwoa@ Horzzey w. F.MATHEWsoN PULP SCREEN WliedFMal-zelasom y Mcm May 10, 1955 Filed April1, 1955 May 10 1955 l w. F. MATHl-:wsoN 2,707,905

PULP SCREEN Filed April l, 1953 9 Sheets-Sheet 6 May 10, 1955 w. F.MATHEwsoN PULP SCREEN 9 Sheets-Sheet '7 Filed April l, 1953 Iaweizrrf.-Wifafed May 10, 1955 w. F.- MATHEwsoN 2,707,905

PULP SCREEN Filed April l, 1955 9 Sheets-Sheet 8 y M ci WM uoaaeg May10, 1955 w. F. MATHEwsoN 2,707,905

PULP SCREEN y Filed April l, 1955 9 Sheets-Sheet 9 United States PatentO PULP SCREEN Wilfred F. Mathewson, Boston,-Mass. Application April 1,1953, Serial No. 346,194

13 Claims. (Cl. 92-35) This invention relates to pulp screens, andespecially to pulp screens of the type shown in my Patents Nos.2,450,838, dated October 5, 1948,` 2,525,701, dated October l0, 1950,and 2,682,812, dated July 6, 1954.

The pulp screen shown in each of said patents and application comprisesa casing to which unscreened pulp is delivered and which encloses ascreening unit provided with two concentrically arranged cylindricalscreening elements forming between them an annular clearance space of asize to permit the acceptable portion of unscreened pulp to passtherethrough, but of insuttcient size to allow the unacceptable portionof such pulp to pass. One of the said two screening elements is providedwith a plurality of inlet slots which extend parallel to the axis ofsaid element and provide passages through which the pulp tiows from themass of unscreened pulp into said clearance space. Said screeningelement is also provided with a plurality of discharge grooves which areparallel to the slots and which are open to the clearance space, theslots and grooves preferably being alternately arranged.

The screening operation involves the passage of the unscreened pulpthrough the inlet slots into the clearance space and the flow of theacceptable portion of said pulp through the clearance space and into thedischarge grooves which deliver the screened pulp into a receivingchamber with which the grooves communicate. One or both of the screeningelements are rotated about the common axis during this pulp-screeningoperation.

In the patents above referred to there is a single screening unit in thecasing, while in the device shown in my Patent No. 2,682,812 there are aplurality of separate, independent screening units located within thecasing and arranged'in circular formation, the screened pulp from eachscreening unit being delivered to a common receiving chamber.

When the pulp screen is in use there is a possibility that during thescreening operation some libres or flocks may get caught on the edge ofthe partition dividing aninlet slot from a discharge groove either atthe point where the inlet slot opens into the clearance space or at thepoint Where the clearance space communicates with the discharge groove.The accumulation of fibres at this point Will clog the clearance spaceand unless the clogging libres are removed, the clearance space maybecome obstructed to such an extent as to materially interfere with thescreening operation.

lt is, therefore, one object of the present invention to provide meansfor progressively flushing the clearance space of the screening unit ina circumferential direction, such ushing operation serving to flush fromthe clearance space any obstruction to the ow of the stock into saidclearance space which may interfere with the screening operation.

In my Patent No. 2,525,701 and also-in my Patent No. 2,682,812 the inletslots are shown as tapered, said slots being widest at the entrance edgeand narrowest at the edge that opens into the clearance space. Withinlet 2,707,905y Patented May 10, 1955 slots having this constructionthere is a possibility that some unacceptable portion of the pulp stockor some llocks resulting from the ilocculation of the pulp may How intothe wider entrance edge of the slot and become lodged therein because ofthe decreasing Width of the slot, thereby clogging the slot andinterfering with the screenlng operation.

It is, therefore, another object of my invention to provide means forperiodically back-Washing the various inlet slots by delivering into thenarrow edge of the slot water under pressure which will serve to flushthe slot and force back out' of theslot any liocks or other materialwhich is clogging the slot, thereby freeing the slot for continuedscreening operation.

Referring now to the drawings, wherein I have illustrated some selected'embodiments of my invention:

Fig. l is a plan View of a pulp screen embodying the invention with aportion broken out.

Fig. 2 is a section through the upper part of the casing on the line 22, Fig. 1.

Fig. 3 is a'section through the lower on the same line.

Fig. 4 is a section on the line 4-4, Fig. 3.

Fig. 5 is an enlarged cross sectional view through one of the screeningelements. v

Fig. 6 is a section on the line 6 6, Fig. 5.

Fig. 7 is a section similar to Fig. 2 but illustrating a diilerentembodiment of the invention.

Fig. 8 is a section similar to Fig. 3 but illustrating said differentembodiment of the invention.

Fig. 9 is an enlarged section on the line 9--9, Fig. 7.

' Referring first to Figs. l to 6, 1 indicates a casing having in itsupper end an inlet opening 2 through which unscreened pulp is deliveredto the interior of the casing, and also having located therewithin aplurality of separate screening units 3, said screening units beingillustrated as being arranged in circular formation, as shown best inFig. Y1.

Each screening unit is of the type illustrated in my above mentionedpatents and application, and comprises two concentrically arrangedscreening elements 4, 5 having a common axis and providing between thema clearance space 6 which is of a size to permit acceptable portions ofthe pulp stock to pass therethrough but which is insuilicient in size toallow unacceptable portions of the stock to pass. The outer screeningelement 4 is provided with a plurality of vertical inlet slots 7 whichlead from the exterior of the screening element into the clearance space6, and said outer screening element also is provided with a plurality ofdischarge grooves 8 which communicate with the clearance space andextend parallel part of the casing to the inlet slots 7. The inlet slotsand discharge grooves are preferably arranged alternatively as best seenin Fig. 5.

As stated above, the screening operation involves the flow of the stockto be screened through the inlet slots 7 into the clearance space 6, andfrom the clearance space into the discharge grooves 8, all of whichcornmunicate with a receiving chamber into which the screened stock isdelivered.

In the construction shown in Figs. l-6, the outerscreening element 4 isstationarily mounted and the inner screening element 5 is rotativelymounted. The outer screening element of each screening unit is providedat its upper end with a ange 9 which is anchored to an upper partition93 with which the casing 1 is provided.

The upper end 11 of the inner screening element S is of a size to litthe interior of the outer screening element 4, said screening element 4thus providing a bearing for the upper end of therotating innerscreening element 5. The clearance vspace 6 above referred to does notextend clear to the upper end of the screening units,

said clearance space terminating at its upper end just bclow the bearingportion 11.

The lower end of the inner screening element 5 is provided with a hollowshaft 12 which depends through a bearing 13 carried by a lower partition14 with which the casing 1 is provided. The lower end 15 of the innerrotary screening element has a. bearing in the lower end of the outerscreening element 4, the clearance space between said elementsterminating at its lower end just above the bearing portion l5.

Since the inner screening element has a bearing in thc outer screeningelement at both the top and the bottom. thereof, the clearance space 6between said elements will always be maintained at a constant dimension.

As stated above, the inner screening element is a rotary element and toprovide for its rotation the hollow shaft 12 has mounted thereon a gear16 which meshes with a large gear 17 rotatably mounted on a stud 18which is secured at its upper end in the partition 14 and in its lowerend in a partition 19 which forms the bottom of a gear chamber in whichthe gears are enclosed. The large gear 17 is driven from a suitablemotor 20 which is mounted on a supporting bracket 21 secured to thecasing, the shaft of said motor being belted by driving belts 22 to apulley 23 carried by a drive shaft 24 that is mounted in suitablebearings carried by the casing. The shaft 24 has a small pinion 25thereon which meshes with and drives the gear 17.

The gear 17 is centrally located and it meshes with and drives the gear16 for each of the screening units, as best seen in Fig. 4, said gear 17thus being common to all of the screening units.

The hollow shaft 12 for each screening unit is shown as connected to itsinner screening element 5 through the medium of a driving pin 25 whichis mountedl in a ange 26 with which the shaft 12 is provided and theupper end of which is received in a socket formed in the lower end ofthe screening unit 5.

The outer screening unit 4 of each screening clement is yieldinglymounted in the casing so as to allow for a certain amount of lateralmovement during the rotation of the inner screening element providedthere is even the least deviation between the axis of the shaft 12 andthe axis of the inner screening unit 5. For this purpose, the anchoringpins 10 for each outer screening element 4 occupy slots 27 in the ange9, and the lower end of the outer screening element is provided with aange 2S which is surrounded by a ring 29 of rubber or other yieldingmaterial that is confined between two annular plates 30.

By thus yieldingly mounting the outer screening element, the latter willhave sufficient lateral movement to prevent any binding action betweenthe screening elements in case there is any disalinement between theaxis of the inner screening element 5 and the axis of the shaft 12.

The discharge grooves S of all of the screening units 3 open at theirupper ends into an upper receiving chamber 31 which is located betweenthe upper partition 93 and the top 32 of the casing, said upperreceiving chamber 31 communicating with a vertically' extending space 33through an opening 34, said space 33 also communieating with a screenedpulp discharge opening 35. The discharge grooves S open at their lowerends into a lower receiving chamber 31 which is common to all thescreening elements, and said chamber 31 communicates through an opening36 with the lower end of the vertical passage 33, as shown in Fig. 3.

By this means the screened pulp from all of the screening units isdelivered into the common receiving chambers 31, and from said chambersis delivered to the screened pulp discharge opening through the verticalpassage 33.

If desired, I may provide the inner and outer screen ing elements with acentral bearing, as shown at 37 in Fig. 2, and where this constructionis employed the Cil central portion of the outer screening element willhave a yielding mounting of the character used for the lower endthereof. As shown in Fig. 2, the central portion of the outer screeningelement has two spaced tlanges 38, cach of which is encircled by acushioning ring 39 held between two annular plates 40.

The discharge grooves 8 of the outer screening element are divided intotwo sections, an upper section above the central bearing 37 and a lowersection below said central bearing. The casing is provided with anauxiliary receiving chamber 41 into which the lower ends of the uppersections of the discharge grooves open and into which the upper ends ofthe lower sections of the discharge grooves likewise open, so that thescreened pulp will be delivered into this auxiliary receiving chamber4l. Said chamber 41 opens into the vertical space 33 through a dischargeopening 42, as seen best in Fig. 2.

This inner rotary screening element 5 is provided with a central chamber43 which communicates at its lower end with the opening 44- through thehollow shaft, as shown in Fig. 3, and said inner screening element isalso provided with a plurality of liushing chambers 45 which extendlongitudinally ofthe screening element. Said inner screening element 5is further provided in its periphery with vertically extending flushinggrooves 46 that open into the clearance space 6, said grooves alsocommunicating with the chamber 43 through openings 83.

Means are provided for maintaining water under pressure in the flushingchambers 45, and there is provided jet openings 47 between the flushingmembers 45 and the llushing grooves 46. As illustrated in Fig. 5, thereis a ushing groove 46 between each two adjacent flushing chambers 45,and each chamber 4S communicates with the adjacent llushing grooves 46through the jet openings 47.

Means for maintaining the llushng chambers 45 filled with water underpressure will be presently described.

With this construction it will be observed that the jets of water underpressure issuing from the jet apertures 47 will serve to ush theclearance space 6, and since the inner screening element 5 is rotating,this ushing action through each flushing groove 46 will be carried onprogressively in a circumferential direction. Such continuousprogressive flushing will serve to keep the clearance space clear fromany liock or obstruction that tends to accumulate at the point where theinlet slots 7 open into said clearance space.

The material flushed [rom the clearance space, which constitutes therejects and tailings, will pass into the chamber 43 through the openingsS3. From said chamber the rejects and tailings flow through the shaft 12into the chamber 87 and are drained off through the drain pipe 88.

The upper end of each inner screening element is closed by a cap memberwhich is rigidly secured thereto, as shown in Fig. 6. Said cap isprovided at its upper end with a hub 49 which extends into the lower endof a fixed support member 50 which has a through opening, an annular'cushion seal member 51 being interposed between the hub 49 and themember Sii. The member 50 for each screening unit has connected theretoone end of a U-shaped supply pipe 52, the other end of said supply pipeleading into an annular chamber 53 surrounding the top of the casing.This chamber 53 is supplied with water under pressure through an inletopening 54. F[he head 48 of each screening unit is provided withinterior chambers 55 which communicate with the various flushingchambers 45. By this means all the flushing chambers 45 of each of thescreening units is supplied with the flushing water under the desiredpressure.

In addition to the means for progressively ushing the clearance space ina circumferential direction, there is also provided means forback-washing the inlet slots 7 to flush backwardly out of each slot anyock or accumulation of unacceptable stock which may enter the ajozsoWide end of any slot and become lodged therein .because of theprogressively decreasing width. of the slot. For this purpose the innerscreening element is provided with a back-washing slot 55 which issimilar to the flushing slots 46 but has no communication with thechamber 43. This back-Washing slot 55, however, communicates through jetopenings 56 with the adjacent flushing charnbers 45, said jet openings56 having a larger diameter than the jet openings 47. As the innerscreening element rotates, the back-washing slot 55 will moveprogressively past the various inlet slots, and as said flushing openingcomes into registry with any inlet slot the jets of Water under pressuredelivered through the opening 56 will provide a back-washing ow of Waterthrough the clearance space and into the inner narrow end of the inletslot, and thereby any material which may have become lodged in saidinlet slot will be forced back through the slot into the space outsideofthe screening unit.

This back-Washing slot 55 operates to back-wash the inlet slotssequentially, and hence each slot will have a back-washing operation foreach rotation of the inner screening element 5.

In the construction shown in Figs. 1 6 the material which is back-washedfrom any inlet slot is delivered into" the mass of unscreened pulpwithinthe casing.

In Figs. 7-9 I have shown a different embodiment of the invention inwhich a back-wash receiving chamber is provided to receive the back-washfrom each inlet slot. 1n the construction shown in Figs. 7-9, the outerscreening element is mounted for rotation and the inner screeningelement is held stationary. The device of Figs. 7-9 is also providedwith a stationary back-wash receiving chamber located exteriorly to therotating outer screening element and with which the inlet slots comeinto registry sequentially as the outer screening element rotates. Theback-washing groove of the stationary inner screening element is locatedopposite the back-wash receiving chamber so that as the various inletslots move between the back-wash groove and the back-wash receivingchamber, the materialin the inlet grooves is forced into the back-washchamber.

In the construction shown in Figs. 7-9 the outer screening element isindicated at 4a andthe inner screening element at 5a. In thisconstruction the outer screening element 4a is secured to the hollowshaft 12 by means of a coupling pin 60, and the head 48a of the innerscreening element is secured to the supporting member 50a. The shaft 12is rotated by the same drivingmeans above described and illustrated inFigs. 3 and 4.

Located exterior to the rotating outer screening member 4a of eachscreening unit is a `back-wash receiving member 61 having a centralvertically extendngchamberl 62 and provided with lip portions 63 whichbear against the exterior of the rotating outer screening element 4a andform between them a slot 64 which opens into the chamber 62'. In thisembodiment the stationary inner screening element 5a is' provided with avertically extending back-washing slotva similar to the slot 55 of Fig.5, said slot being connected to one of the flushing chambers 45 througha jet opening 65.

As each inlet slot 7a passes between the flushing groove 55a and thethroat 64, the force of the Water delivered through the jet openings 65will establish a back-washing current from the back-washing groove 55aJthrough the slot into the chamber 62,` so that as each inlet slot isbackwashed the discharge from said slot is delivered into theA back-washreceiving chamber 62.

It will be understood that `there is a similar back-wash receivingchamber for each .ofthe screening units 3 and each back-wash receivingchamber 62 has communication with a central 'chamber 66 so that theback-wash from each of the screening units is delivered'into saidchamber 66. Said chamber has a discharge pipe 67 extending from itslower end through which the accumulated back-wash may be discharged,said`pipe being 6 controlled by a suitable valve 68. For connecting eachback-wash receiving chamber 62 to the central chamber 66 there isprovided a series of pipe connections 69.

In order to provide a proper contact of the lips 63 with the exterior ofthe rotating outer screening element 4a I have provided a mounting forthe back-wash receiving chambers which includes springs 70 thatyieldingly press the back-wash receiving member toward the screeningelement. 61 is provided with pins 71 which extend through lugs 72carried by the casing, said spring 70 being confined between the lugsand the member 61.

Where the screening elements are provided with a central bearing and thedischarge grooves and inlet slots are formed in upper and lowersections, I propose to employ an upper and a lower back-wash receivingmember for each screening unit, the upper back-wash receiving memberdelivering into the upper end of the central chamber 66 and the lowerback-wash receiving chamber delivering into the lower portion of saidchamber 66, as shown in Fig. 8.

I claim:

l l. A pulp screen comprising two concentrically ar-v ranged screeningelements forming between them an annular clearance space of a size topermit acceptable portions of pulp stock to pass therethrough but ofinsuflicient size to permit unacceptable portions of said stock to passtherethrough, the outer screening element having a'plurality of inletslots leading to said clearance space through which pulp stock to bescreened is delivered thereto, and also having a plurality of dischargegrooves extending parallel to the slots and open to said clearancespace, whereby acceptable portions of the pulp stock may pass throughthe inlet slots to the discharge grooves through the clearance space, achamber communicating with all of said discharge grooves and into whichthe screened stock is delivered through said grooves, means for rotatingone of said screening elements about its axis, the inner screeningelement having an interior tailings-receiving chamber which communicateswith the clearance space, and means separate from the tailings-receivingchamber to deliver jets of llushing water into said annular clearancespace progressively in a circumferential direction while maintaining thenormal forward flow of pulp stock through the inlet slots and the normalilow of tailings into the tailingsreceiving chamber.

2. A pulp screen as defined in claim l in which the inner screeningelement has a llushing chamber separate from the tailings-receivingchamber and extending longitudinally thereof and means are provided forsupplying said chamber with water under pressure, and in which the innerscreening element has a plurality of jet openings leading from theflushing chamber and adapted to deliver water under pressure from saidflushing chamber into the clearance space, thereby flushing -therefromany pulp material which has become lodged therein while maintaining thevnormal forward flow of pulp stock through the inlet slots.

3. A pulp screen as dened in claim 1 in which the inner screeningelement has a central chamber and la slot which provides communicationbetween said chamber and the Vclearance space, and means are provided todeliver jet's of water under pressure into said slot thereby to flushinto said chamber any ilocks or unacceptable portions of the pulp stockwhich may have become lodged in the clearance space.

4. A pulp screen as dened in claim l in which the inner screeningelement has a central tailings-receiving chamber and a plurality ofslots, each of which provides a passageway between the clearance spaceand said chamber, and which includes means for delivering jets of waterunder pressure into each slot thereby to flush into said chamber flocksand unacceptable portions of thepulp stock which may have become lodgedin thel clearance space while maintaining the normal forward Eachback-wash receiving member tlow of pulp stock through the inlet slotsand the normal tlow of tailings into the tailings-receiving chamber.

5. A pulp screen as dencd in claim l in which the inner screeningelement has a central tailings-receiving chamber and also has aplurality of ushing chambers, each of which is connected to theclearance space through a plurality of jet openings, and means areprovided to supply the llushing chambers with water under pressurewhereby the jets of water delivered from the jet openings flush fromsaid clearance space into thc central tailings-receiving chamber anyportions of the pulp stock that become lodged in the clearance space.

6. A pulp screen comprising two concentrically arranged scrceningelements forming between them an an nular clearance space of a size topermit acceptable portions of pulp stock to pass therethrough but ofinsuicient size to permit unacceptable portions of said stock to passtherethrough, the outer screening element having a plurality of inletslots leading to said clearance space through which pulp to be screenedis delivered thereto, and also having a plurality of discharge groovesextending parallel to the slots and open to said clearance space,whereby acceptable portions of the pulp stock may pass through the inletslots to the discharge grooves through the clearance space, a chambercommunicating with all of said discharge grooves and into which thescreened stock is delivered through said grooves, means for rotating oneof said screening elements relative to the other, said inner screeningelement having a tailings-receiving chamber which communicates with theclearance space, and means separate from the tailings-receiving chamberto deliver water under pressure into the delivery side of the inletslots successively thereby to back-wash the slots sequentially and cleartherefrom any flocks or unacceptable portions of the pulp stock that maybecome lodged therein.

7. A pulp screen as defined in claim 6 in which the means for deliveringthe water under pressure into the delivery side of cach slot is carriedby the inner screening element.

8. A pulp screen as defined in claim 6 in which the water under pressureis delivered into the delivery side of the slots one at a time andsuccessively.

9. A pulp screen comprising two concentrically arranged screeningelements forming between them an annular clearance space of a size topermit acceptable portions of pulp stock to pass therethrough but ofinsufficient size to permit unacceptable portions of said stock to pass,the outer screening element having a plurality of inlet slots leading tosaid clearance space through which pulp to be screened is deliveredthereto, said slots being wider at the input side than at the deliveryside that opens into the clearance space, said outer screening elementalso having a plurality of discharge grooves extending parallel to theslots and open to said clearance space, whereby acceptable portions ofthe pulp stock may pass through the inlet slots to the discharge groovesthrough the clearance space, a chamber communicating with all of saiddischarge grooves and into which the Screened stock is delivered throughsaid grooves, means for rotating the outer screening element, means fordclivering water under pressure into the narrow delivery sides of thevarious slots successively to back-wash said slots sequentially andclear them from any ocks or unacceptable portions of pulp stock that mayhave become lodged therein, and a stationary back-wash receiving chambersituated exterior to the outer screening element closely adjacentthereto and in position to receive the back-wash from each slot as itpasses said member.

10. A pulp screen comprising two concentrically arranged screeningelements forming between them an annular clearance space of a size topermit acceptable portions of pulp stock to pass therethrough but ofinsuicient size to permit unacceptable portions of said stock to passtherethrough, the outer screening element having a plurality of inletslots leading to said clearance space through which pulp to be screenedis delivered thereto, and also having a plurality of discharge groovesextending parallel to the slots and open to said clearance space,whereby acceptable portions of the pulp stock may pass through the inletslots to the discharge grooves through the clearance space, a chambercommunicating with all of said discharge grooves and into which thescreened stock is delivered through said grooves, means for rotating theouter screening element about its axis relative to the inner screeningelement, means carried by the inner screening element to deliver waterunder pressure through the clearance space into the delivery side ofeach inlet slot in succession, thereby to back-wash the slotssequentially and clear therefrom any flocks or unacceptable portions ofpulp stock that may have become lodged therein, and a stationaryback-wash receiving member situated exterior to the outer screen elementclosely adjacent thereto and having an elongated throat which isparallel to the inlet slots and which is in position to receive theback-wash from each slot as it passes said throat during the rotationofthe inner screening element.

11. A pulp screen as defined in claim l0 in which the inner screeningelement is stationary and has a groove in its periphery which parallelsthe inlet slots in the outer screening element, and which has means todcliver water under pressure to said groove whereby as each inlet slotof the outer screening element passes the groove during the rotation ofthe outer screening element, water under pressure will be delivered intothe slot thereby to back-wash it and clear it from any flocks orunacceptable portion of the pulp stock that may have bccome lodgedtherein.

l2. A pulp screen as defined in claim 11 and which includes a back-washreceiving chamber situated exteriorly to the outer screening element andhaving an elongated throat which parallels the inlet slots of the outerscreening element and is located opposite the groove in the innerstationary screening element, whereby as each inlet slot of the outerscreening element comes into registry with both said groove of the innerscreening element and the throat of the back-Wash receiving charnber,water under pressure will be delivered into the delivery side of saidinlet slot, thereby back-Washing the latter and delivering the back-washinto the throat of the back-wash receiving member.

13. A pulp screen comprising a casing having an inlet opening throughwhich unscreened pulp stock is delivered, a plurality of screening unitssituated within said casing, each screening unit comprising twoconcentrically arranged cylindrical screening elements forming betweenthem a clearance space of a size to permit I acceptable portions of thepulp stock to pass therethrough but insufficient in size to permitunacceptable portions of said stock to pass, the outer screening elementof each screening unit having a plurality of inlet slots extendingtherethrough and communicating with the clearance space thereof, andalso having a plurality of discharge grooves open to the clearancespace, said casing having a chamber with which all of the dischargegrooves of all of the screening units have communication and into whichthe screened pulp from all of said grooves is delivered, means to rotatethe outer screening element of each screening unit, the inner screeningelement of each unit being stationary and having an interiortailings-receiving chamber which communicates with the clearance space,means separate from the tailings-receiving chamber to deliver jets offlushing water into the annular clearance space of each screening unit,a back-wash receiving member for each screening unit located exterior toand closely adjacent the outer screening element, each back-washreceiving member having a throat which communicates with cach slot ofthe outer screening member as it ro- 9 taies, means to deliver ushingwater under pressure into each inlet slot as it passes the throat of thebackwash receiving member, and a collecting chamber cornmon to al1 theback-Wash receiving members for co1- lecting the tailings deliveredthereinto by the ushing operation.

References Cited in the le of this patent UNITED STATES PATENTS2,525,701 Mathewson Oct. 10, 1950

